A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as “Tetra Brik” or Tetra Brik Aseptic (registered trademarks), which is made by folding and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material comprises a layer of barrier material, e.g. aluminum foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with one or more layers of heat-seal plastic material eventually forming the inner face of the package contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, after sterilization, is removed, e.g. evaporated by heating, from the surfaces of the packaging material; and the web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled continuously downwards with the sterilized or sterile-processed food product, and is sealed and then cut along equally spaced cross sections to form pillow packs, which are then fed to a folding unit to form the finished, e.g. substantially parallelepiped-shaped packages.
More specifically, the pillow packs substantially comprise a parallelepiped-shaped main portion; and opposite top and bottom end portions projecting laterally on opposite sides of the main portion and defining respective triangular end flaps to be folded onto the main portion.
A longitudinal sealing strip, formed when sealing the packaging material to form the vertical tube, extends along the pillow packs; and the end portions of each pillow pack have respective transverse sealing strips perpendicular to the relative longitudinal sealing strip and defining respective end tabs projecting from the top and bottom of the pack.
The end portions of each pillow pack taper towards the main portion from the respective end tabs, and are pressed towards each other by the folding unit to form flat opposite end walls of the pack, while at the same time folding the end flaps onto respective walls of the main portion.
Packaging machines of the above type are known, in which the pillow packs are folded to form the parallelepiped-shaped packages by means of folding units substantially comprising a conveyor for feeding the packs along a forming path; a number of folding members located along the forming path and interacting with the packs to flex the packaging material along preformed fold lines; a heating assembly which acts on the flaps of each pack to be folded to heat seal the flaps to respective walls of the pack; and a final pressure device which cooperates with each pack to hold the flaps on the relative walls as the flaps cool.
Because of the normal position in which the pillow packs are fed to the folding unit, and the forward movement of the conveyor through the folding members, the end tabs are folded “naturally” onto the relative end walls of the packs, onto the side without the longitudinal sealing strip, i.e. in the opposite direction to the travelling direction of the conveyor.
Though extremely easy to perform, the above method of folding the end tabs reduces the space left on the top end walls of the packages to apply reclosable opening devices.
As is known, opening devices cannot be applied to the sealing areas of the packages, because of the problems posed by heat sealing the opening devices onto uneven surfaces, and to avoid impairing the seals on the packages themselves.
As a result, opening devices can only be applied to the small flat areas adjacent to the sealing strips on the top end walls of the packages, which obviously limits the maximum size of the opening devices.
This limitation is further compounded in view of the increasing number of food products of different physical characteristics packaged as described above, i.e. in packages made of paper packaging material. In particular, certain food products, especially semiliquid products or products containing fibre or particles, necessarily call for larger opening devices to permit correct pour-out of the product with no clogging.
To increase the space on the packages in which to apply the opening devices, it has been proposed to invert the direction in which the end tab is folded onto the relative top end wall, by easing it onto the same side as the longitudinal seal.
The folding direction of the end tabs of the packs is normally inverted by gradually deforming the tabs by means of appropriately shaped contrasting surfaces, against which the packs slide as they travel along the forming path.
Though advantageous in many respects, the above method of folding the end tabs of the packs fails to ensure total repeatability and reliability of the folding operation.